Not long after the advent of engine powered vehicles, it became important to install devices in or on such vehicles for the purpose of measuring and indicating forces due to changes in velocity. It is important today to do so. One good reason for collecting such information is to establish a base of knowledge about stress and strain in structural elements of a vehicle. For example, a history of changes in direction and velocity relative to time for an aircraft during flight can be used in conjunction with a knowledge of the dimensions and materials of construction of the aircraft and the equations of motion in engineering science to calculate stresses and strains in individual framing elements and fasteners. Such knowledge is vital is design.
The rate of change of velocity with respect to time for any object is known as acceleration, and is measured in distance per unit time per unit time, such as meters per second per second. To cause a body to exhibit an increase in velocity in a straight line over a period of time, it is necessary to exert a force on the body in the direction of motion. The relationship between the force exerted and the acceleration of the body is the well known equation: EQU F=m.times.a
where F is the force, m is the mass of the body, and a is the acceleration. Exerting a force on a moving body in the direction of motion will normally cause a body to accelerate, and likewise, exerting a force on a moving body opposite to the direction of motion will cause the velocity of the body to decrease as a function of time. Forces, velocities and accelerations in engineering calculations are vector quantities, having both magnitude and direction, and a convention is usually established concerning the direction of positive vectors. If, in the example given, the direction of motion is regarded as the positive direction, then a force applied opposite to the direction of motion may be considered a negative force, and a decrease in velocity with respect to time experienced by the object in question as a result of the negative force may be considered a negative acceleration. For purposes of calculation, the concept of negative acceleration is more convenient than deceleration.
Another example of the importance of devices to measure acceleration is the case of the modern automobile. Without, for the moment, considering the forces involved with changes in direction as an automobile rounds curves or corners, considering only an automobile traveling in a straight line, it is clear that the engine and drive train of the automobile must exert a force in the direction of motion to cause the automobile to gain velocity, and that the time rate at which velocity is gained is a function of the ability of the engine and drive train to exert the force. Moreover, if the engine and drive train at one particular time of testing is capable of providing an acceleration of a certain magnitude, the magnitude of acceleration provided at another time of testing may be used in comparison to the former test to gage the relative efficiency of the engine and drive train.
In a like manner, the retarding force (negative force) exerted on the same vehicle by its braking system will cause the velocity to decrease as a function of time, and the measured negative acceleration taken in more than one test (or compared to a standard) may be used to gage the relative efficiency of the braking system.
Only positive and negative acceleration in straight line motion has been thus far discussed in this specification. Forces are involved in changes of direction of velocity as well as changes in magnitude of velocity. An automobile rounding a corner, for example, may maintain the same magnitude of velocity (speed) throughout the maneuver and continue then in a straight line, at the same rate of motion as before. The velocity of the automobile, however, changes in direction, and force is involved as well as acceleration. The driver of the automobile causes force to be applied to the automobile by turning the front wheels, and a friction force between the tires and the road surface is exerted on the automobile. The tighter the turn and the higher the speed at which the turn is executed, the higher the force. At some point, the force required to maintain the turn may exceed the friction between the tires and the road surface, and the automobile, attempting to maintain straight line motion, will skid.
Forces involved in turning are called centrifugal or centripetal forces. The deviating force acting on a body in curvilinear motion around an instantaneous center of rotation is called the centripetal force. In the case of an automobile rounding a corner, the force exerted by the road surface on the tires of the automobile is the centripetal force. The equal and opposite force exerted by the body on the medium of restraint is called the centrifugal force. The force exerted by the tires on the road surface is the centrifugal force. In the system of a ball on a string being whirled in a circle, the force exerted by the string on the ball, acting toward the instantaneous center of rotation, is the centripetal force. The forces involved in turning are necessary to provide acceleration toward the instataneous center of rotation so that turning may be maintained.
There is another acceleration that needs to be discussed to some degree in the proper understanding of accelerometers. This is acceleration due to the universal forces of gravitational attraction. The forces exerted as a result of this natural phenomenon by one body upon another on the Earth's surface are so small as to be negligible for the purposes of understanding accelerometers, but the force exerted on objects by the Earth, known as the gravitational force, is appreciable, and must be taken into account. This force is what we know as the weight of an object, and is an object's mass multiplied by the acceleration of gravity, after the formula F=m.times.a. In this case W=m.times.g, where W is weight and g is the "standard" acceleration of gravity on the Earth's surface. Disregarding minor local variations due to altitude and the known flattening of the Earth at the poles, g may be taken with reasonable accuracy as 9.81 meters per second per second; which is equal to 32.174 ft. per second per second.
There have been many devices designed to indicate acceleration for a large number of purposes, and patents have been issued. These devices are generally known as accelerometers. All such devices make use of a mounting by which the device may be fastened to the body for which acceleration information is desired, and at least one acceleration sensitive element that will issue some measurable indication relative to experienced acceleration. Steel balls traveling in grooves on inclined planes have been used, for example, with the length of the grooves oriented in the direction of expected motion of the body, and the inclination of the planes downward in the expected direction of travel. A steel ball will roll up such an inclined groove to a height corresponding to the magnitude of acceleration, and the length along a groove may be marked to indicate the acceleration in some convenient terms, such as meters per second per second, or miles per hour per second. Bubbles in curved glass tubes have been used, and the height to which a column of liquid may rise (or fall) in vertical legs of tubing having a connecting passage is another convenient acceleration sensitive element. The use of a pendulum of one sort or another as the acceleration sensitive element has also been popular.
The particular construction, the choice of sensitive element and the way an accelerometer presents information and, perhaps, records it for later use, is strongly influenced by the general use to which an acceleromter might be put. An accelerometer, for example, developed for use in a nuclear submarine, might be a complicated and relatively expensive device; while an instrument to be attached to a shipping crate to record the history of handling of the crate, might be expected to be a more simple and relatively less expensive device, sacrificing, perhaps, some of the more esoteric functions of the instrument meant for the submarine.
One of the uses to which a recording accelerometer might be put, would be to record the acceleration effects over a period of time on a vehicle as a means of monitoring the performance of the operator of the vehicle. One situation in which such an instrument might be desireable would be in cases where one person operates a vehicle owned by another. The granting of the privilege of operating a vehicle is quite often made on the implicit or explicit understanding that the vehicle will be operated in a safe manner. This is the case, for example, in a family situation, where one person may own and be responsible for a vehicle, such as a passenger car, and frequently assign its use to another family member, such as a son or daughter, on the understanding that the vehicle is to be used in a safe manner. This situation also pertains where a company owns a number of cars or trucks, and employees of the company are assigned to drive them.
In these cases of assigned operation with implied or even written agreement as to modes of operation, it would be greatly to the benefit of the responsible owner and/or insurer of such vehicles to have the use of an instrument that could be mounted on the assigned vehicles, with or without the operator's knowledge, and would provide indication of the manner in which the vehicle is operated. Such an instrument need be fairly repeatable and accurate. It should also be small and easily mountable on a vehicle either inside or outside the passenger compartment, or even in the engine compartment. The device would need to record the maximum accelerations experienced in the direction of motion of the vehicle, which would indicate the manner in which an operator increases the vehicle speed and the manner also in which the operator applies the brakes. It should also, either in the same or in a similar instrument, be able to record the maximum accelerations at right angles to the direction of travel of the vehicle, which will tell the manner in which the operator handles the vehicle in cornering. These readings, then, could be accessed by the responsible owner and/or insurer, and compared to standards for performance. The ability to discern irresponsible handling of such a vehicle would enhance public safety and provide a potential savings for the owner and/or insurer, by allowing greater and more accurate discrimination.
The instrument needs also to be rugged and reliable, and should preferably be inexpensive, avoiding the use of electrical and computer systems for operation. Many prior instruments that might be used for such a purpose use multiple elements and many operating parts, and are therefore expensive to fabricate and relatively unreliable in use, Some, such as those incorporating tubing and liquids, may be fragile, and unreliable as a result. Another common fault of accelerometers is the fact that acceleration-sensitive elements are generally not linear in indication. Given the case of a pendulum type instrument, for example, the number of degrees that a pedulum swings through as a result of being acted upon by an acceleration, is a trigonometric function of the magnitude of the acceleration. This fact makes the scale calibrations progressively more difficult to read as higher and higher values of acceleration are measured. What is needed is a rugged, inexpensive instrument that may be conveniently mounted on a vehicle in a variety of ways, and which will provide an easily readable indication of the maximum accelerations experienced over a period of time, but that may also be used to provide a continuous and easily readable indication of acceleration as experienced.